There are certain situations where you can early withdraw your MPF:
(1) Account balance less than $5000 and did not make any contributions in the past 12 months, and WILL NOT make any contributions in foreseeable future
(2) Going overseas and will permanently leave HK
(3) Declare that you are going to retire at age 60 and will not work anymore afterwards
Therefore your mum shall withdraw the MPF at age 60 the earliest. Or if she fulfills the conditions (1) or (2), she can immediately request for it.